100 years has passed since it was first proven that universe extends far beyond the Milky Way

Is the Milky Way the lone one that exists in the Cosmos? Or does Cosmos extend far beyond Milky Way? We are now absolutely certain that the second answer is correct. However, it was proven for the very first time just 100 years ago when astronomer Edwin Hubble read a paper on the distance to the Andromeda Nebula at a meeting of AAS (the American Astronomical Society).

1 January is the day when everybody celebrates the commencement of the New Year, very few people think about boring work and it seems that no one would think of having any meetings. Nevertheless, on 1 January a hundred years ago, a meeting of AAS was held, at which Edwin Hubble, an astronomer not very well known at the time, told how he had measured the distance to the Andromeda Nebula.

From the description it may seem that it was something unexciting, but actually, it was then that mankind finally became convinced that the Cosmos was much bigger than it appeared, and the modern picture of the world, in which the universe is filled with hundreds of billions of galaxies, started to take hold.

It was Edwin Hubble’s research that showed that the Cosmos was not limited just to the Milky Way alone. But in order to properly understand how significant that report that Hubble delivered on 1 January 1925 was, one needs to know how notions about the universe have changed over the past centuries.

From the Solar System to the Galaxy

Six centuries back, in the imagination of most astrophysicists, the Cosmos was extremely small by modern standards. At its centre was the Earth, around it orbited the planets, the Moon and the Sun, and beyond them was a sphere on which hung the fixed stars. When Copernicus proved in the first half of the 16th century that it was not the Earth but in fact the Sun that was at the centre of everything, for all the significance of this discovery, essentially not much changed because the Cosmos remained confined to the Solar System.

Real revolution in the ideas about the universe imperceptibly took place in the 18th century. In 1718, the English astrophysicist Edmond Halley methodically studied astronomical records for several centuries and exposed the fact that the stars are not stationary, and in the subsequent decades, researchers established that they can have dissimilar luminosity and be at diverse distances from us.

At first glance, it appeared that scientists had thrown humanity out of a stable and comprehensible Cosmos, which had the Sun as its center, into complete anarchy with a bunch of incomprehensible objects, and the Sun was just one among them. Nevertheless, even in this colossal chaos, they found a pattern quickly. Back in 1616, Galileo Galilei had proved that the bright band of the Milky Way is a cluster of very small stars. And further observations disclosed that the number of luminaries rises near it.

All this let in the middle of the 18th century Thomas Wright and later on Immanuel Kant to conclude that the stars around us form a single system in the form of a flat disc. And in 1784-1785 William Herschel was able to calculate the number of stars in different parts of the sky even, to establish the size of this disk. He also found out that the solar system is near the center of this very disc. As we know very well now, he was erroneous about both the first and the second, but the concept of the “starry island” turned out to be right and became fixed in people’s perception.

Nebulae and the problem of distances in space

Still, there were enough objects of unintelligible nature in space. And the most inexplicable were light spots with not always clear boundaries, which were termed nebulae. The one in the constellation of Andromeda stood out amongst them. Even Kant and Halley speculated that it might be similar to the Milky Way. But, unlike the latter, it was actually not possible to identify individual stars in it for a long time. And this gave rise to the questions that whether all nebulae are of the same nature, where they are positioned relative to the Milky Way: inside it, or are separate systems.

In 1864, researchers studied the spectra of nebulae and found that there are at least two kinds of nebulae: those whose spectrum is totally different from the stellar spectrum, and those that bear a resemblance to luminaries. The Andromeda Nebula belonged to the latter. In 1885, a supernova erupted in the Andromeda Nebula, which should have ostensibly confirmed the extragalactic nature of this object, but astrophysicists misidentified its type. As a result, the distance to it was estimated to be merely 17 thousand light years, which meant that it was within the Milky Way.

In general, the problem of determining distances in space has endured as one of the biggest hindrances in the development of ideas about our place in it. If you observe only a group of lights and darkness between them, how do you know which ones are far and big and which ones are near and small? Researchers already knew that precise measurement of the stars’ motion could be of help. The same technique that Halley used to “break” the “sphere of fixed stars”. Though, this method was very well suited for luminaries close to the Sun, which shifted by perceptibly large magnitudes. But for something very faraway it gave too uncertain results.

That’s why it was extremely tough to estimate where nebulae with a spiral structure, like the one in the Andromeda constellation, were positioned, even though experts already knew that they consisted of stars. What was required was a reliable method for determining distances, and it soon appeared.

Cepheids and the Great Debate

Cepheids became the key to measurement of large distances in space. As early as the second half of the sixteenth century, astronomers found out that some stars are variable. In 1784, John Goodricke and Edward Pigott found out the variable nature of the star Delta Cephei. Later on, it became clear that variables are of many different types. Stars whose brightness variations were akin to the Delta Cephei began to be termed Cepheids. In the year 1908, the American astrophysicist Henrietta Leavitt, who had been studying variables throughout her life and had discovered an incredible number of them, observed a regularity in the Cepheids. In general, they were all large old pulsating stars, clearly visible at prodigious distances. Nevertheless, their luminosity did vary. And the bigger it was, the bigger the pulsation period was.

That is, as Danish astrophysicist Ejnar Hertzsprung very soon established, knowing the period of a Cepheid, it is very much possible to determine its luminosity, and hence the distance to it. In 1915-1916, this technique was employed to determine distances to Cepheids by the American astronomer Harlow Shapley. The situation of what our Cosmos looked like became more and more perplexing, and finally, in 1920, a discussion of the issue was organized, which came to be known as the Great Debate. The participants were Shapley and another American researcher, Heber Curtis. Interestingly, in this discussion, Shapley himself defended the opinion that nebulae with spiral structure are inside the Milky Way, it has an enormous size – about 300 thousand light years in diameter, and the Solar System is quite far from its center.

Curtis, on the other hand, asserted that the Andromeda Nebula was a separate galaxy, akin to the Milky Way, but that our star system was far smaller than it is, and that the Sun was near the center. Experts argued about just about everything: the dependability of cepheids as a way of estimating distance, the nature of new and supernovae stars, and the gas and dust inside galaxies. In the end, both agreed with some of their rival’s arguments, but in general, each considered himself victorious. It seemed that the issue of what the Cosmos looks like was again far from being solved because each of the two experts had created his own, relatively unswerving model of it.

Edwin Hubble

But at the same time Edwin Hubble, about whom very few people in the scientific world had heard before, was working on the issue. At his disposal was the most powerful telescope of that time with a mirror diameter of 254 cm. And it was owing to it that he discovered a cepheid in the Andromeda Nebula, which everybody was tired of arguing about. The beauty of his discovery was that he did not add anything new to the technique previously used by Shapley. But he used it to disprove the latter’s assertion.

From the measurements, it was inferred that the distance to the Andromeda Nebula was 275 kpc, and thus it is correct to call it the Andromeda Galaxy because it is a detached star system from the Milky Way. True, in fact, at that time this fact was not so extraordinary. In 1922, Ernst Öpik suggested that its flattened shape is result of rotation, and calculated that the distance to it is roughly 450 kpc. And in 1923 Knut Lundmark determined that the distance to this star system is 1 mpc.

Nevertheless, it was Hubble’s measurement that was the final point in the dispute. Shapley was wrong about the place of other galaxies in the overall picture of the Cosmos. Although he was closer to the reality when it came to the size of the Milky Way and the location of the Solar System within it. This is often the case when experts talk about the big picture of the world: they may be wrong about one thing, but right about something else.

By the way, Hubble himself made an error in estimating the distance to the Andromeda Galaxy, underestimating it by about three times. Because it turned out that there are two different types of Cepheids, and each of them has its dependence of period on luminosity. Nevertheless, he was right in the main: the Cosmos is indeed much larger than our Galaxy.